// https://iai.sh.cn/problem/797
#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int n = s.size();
  int ans = 0;
  for (int i = 0; i < n / 2; i++) {
    if (s[i] != s[n - i - 1]) {
      ans++;
    }
  }
  cout << ans;
  return 0;
}